1. Field of the Invention
The present invention relates to a piezo-film speaker and a helmet using the piezo-film speaker, and particularly to a piezo-film speaker capable of efficiently reproducing sound at a high tone quality within a motorcycle helmet requiring a built-in speaker and using the piezo-film speaker.
2. Background Art
Magnet (or voice coil) speakers of a type including a voice coil provided on a truncated cone have been widely used as speakers built into helmets, such as motorcycle helmets. However, magnet speakers according to the background art have limitations in both their failure to permit adequate miniaturization and lightweightness.
One solution is the adoption of a piezo-film speaker within a space of a helmet for improving a rider's comfort when the helmet is put on and worn by the rider. This type of speaker has been disclosed, for example, in Japanese Patent Laid-open No. Sho 63-175106 and the microfilm of Japanese Utility Model Laid-open No. Sho 63-44584.
The above piezo-film speaker, however, suffers from a major shortcoming. Specifically, a flat-plate like piezoelectric material is used as a diaphragm with the above-mentioned piezo-film speaker and as a result, the output (sound pressure) is generally low.
Similarly, Japanese Patent Laid-open No. Hei 7-327298 has disclosed a curved piezo-film speaker characterized in that a higher output efficiency is obtained as a radius of curvature at the curved portion of the speaker becomes smaller.
However, the above-described prior art piezo-film speaker has disclosed only a general technique for increasing the output of the piezo-film speaker, and have ignored tone quality with respect to said speakers. Tone quality is particularly important when the piezo-film speaker is used as a sound speaker and a sound pressure most suitable when it is used for a helmet for a motorcycle rider.
For example, as described in the above document, Japanese Patent Laid-open No. Hei 7-327298, the output of the piezo-film speaker becomes higher as the radius of curvature becomes smaller; however, since the peak of the output characteristic is shifted on a higher frequency side from an audio frequency band, a high tone quality cannot be obtained, particularly, when used for voice or audio reproduction.